In a liquid crystal display device, an active matrix liquid crystal display device in which pixel electrodes are arranged in matrix and transistors are used as switching elements connected to respective pixel electrodes in order to obtain a high-quality image, has attracted attention.
An active matrix liquid crystal display device including transistors, in which metal oxide is used as a channel formation region, as switching elements connected to respective pixel electrodes is already known (Patent Documents 1 and 2).
It is known that active matrix liquid crystal display devices are roughly divided into two kinds: a transmissive liquid crystal display device and a reflective liquid crystal display device.
In a transmissive liquid crystal display device, a backlight such as a cold cathode fluorescent lamp is used, and a state in which light from the backlight is transmitted through a liquid crystal and output to the outside of the liquid crystal display device or a state in which light is not output is selected using optical modulation action of liquid crystal, whereby bright and dark images are displayed. Further, those displays are combined to display an image.
Since a backlight is used in a transmissive liquid crystal display device, it is difficult to recognize display in an environment where external light is strong such as outdoor.
In a reflective liquid crystal display device, a state in which external light, in other words, incident light is reflected at a reflective electrode and output to the outside of the device or a state in which incident light is not output to the outside of the device is selected using optical modulation action of liquid crystal, whereby bright and dark images are displayed. Further, those displays are combined to display an image.
A reflective liquid crystal display device has an advantage in that power consumption is lower as compared to that of a transmissive liquid crystal display device because a backlight is not used, and demand for a reflective liquid crystal display device used for a portable information terminal has increased.
Since a reflective liquid crystal display device utilizes external light, it is suitable for image display in an environment where external light is strong such as outdoor. On the other hand, it is difficult to recognize the display in an environment where light is dim around the liquid crystal display device; that is, external light is weak.